Micro-electro-mechanical systems (MEMS) Fabry-Perot tunable filters are commercially available. These devices typically combine some sort of deflectable membrane, which, in combination with a fixed mirror, defines the tunable Fabry-Perot cavity. The size of the cavity, and therefore the spectral position of the filter's pass band, is modulated or controlled by deflecting the membrane.
Originally, these MEMS Fabry-Perot tunable filter devices were predominately used in wavelength division multiplexing (WDM) applications for telecommunications. More recently, these MEMS Fabry-Perot filters have been deployed in other applications, such as spectroscopy and medical applications, for example.
U.S. Pat. No. 6,608,711 to Flanders, et al. shows one approach for manufacturing the MEMS Fabry-Perot tunable filter. The Flanders device uses a MEMS membrane that is based on silicon-on-insulator technology. Specifically, the membrane is formed by releasing a portion of the silicon device layer from an underlying silicon handle wafer using the intervening oxide layer as the release layer. In the typical implementation, a backside port is provided. A fixed mirror is then bonded to the MEMS membrane die, in order to create the tunable Fabry Perot cavity.
The Flanders device, however, is only one example of MEMS Fabry-Perot filters. Another example is disclosed in U.S. Pat. No. 6,584,126 to Wang, et al., which is based on earlier work by Parviz Tayebati. Specifically, the Tayebati device configuration is based on a vertical cavity surface emitting laser (VCSEL) technology. It uses a high-reflecting (HR) dielectric stack mirror that is suspended on a membrane above a substrate, on which a bottom mirror has been deposited. The Tayebati device has advantages insofar as the device can be monolithically fabricated. It has disadvantages, however, associated with the difficulty in adjusting the size of the Fabry-Perot cavity, among other things.
These MEMS Fabry-Perot filters have been used in various applications. As disclosed in U.S. Pat. No. 6,509,972 to Korn, they can be used in spectrometers. The high finesse, highly-stable optical cavities of the MEMS tunable filters enable highly accurate scanning of the spectral band. See also, U.S. Pat. No. 6,407,376 to Korn, et al. Additionally, as disclosed in U.S. Pat. No. 6,345,059 to Flanders, these MEMS Fabry-Perot tunable filters can be used as the tunable elements in external cavity lasers. Finally, more recently as disclosed in U.S. patent application to Walid Atia, et al., filed on Oct. 17, 2003, assigned Ser. No. 10/688,690, the filters can also be used with semiconductor broadband sources to create a tunable signal that has advantages over tunable lasers in its stability and band of operation.
Additional innovations relative to these MEMS membranes have been disclosed. Specifically, in U.S. patent application Pub. No. US2002/0126726A1 by Flanders, et al., MEMS membranes with integral mirrors or lenses have been produced. In the typical example, a concave mirror is formed on the deflectable membrane. This allows for the formation of a curved-flat or curved-curved optical cavity. The curved-flat cavity allows for the fabrication of a high finesse filter that has lower angular alignment tolerances relative to the input beam.
Moreover, as disclosed in U.S. Pat. No. 6,424,466, to Flanders, multi-cavity, multi-membrane MEMS tunable filters have also been proposed. These combine two opposed MEMS membranes with an intervening suspended highly reflecting mirror. This creates a two-cavity device, which has improved passband performance for some applications. Specifically, the passband is modified from the standard Lorentzian passband of a single cavity for applications such as wavelength routing and also typically has improved side band rejection.